1. Field of the Invention
The present invention relates to a process for treating nitrogenous wastewater and more specifically, to a process for treating nitrogenous wastewater with simultaneous nitrification, autotrophic denitrification, heterotrophic denitrification and COD (chemical oxygen demand) removal.
2. Description of the Related Art
Wastewaters, for example domestic wastewater, livestock wastewater, landfill leachate and industrial water, contain high concentration organic nitrogen compounds and ammonium. In general, organic nitrogen compounds further decomposes to ammonium. Conventional biological processes for treating nitrogenous wastewater shown as FIG. 1 consist of two reaction steps. The first step is aerobic nitrification, in which the ammonium nitrogen is oxidized by chemolithotrophic bacteria into nitrite and further into nitrate, and oxygen is supplied as electron acceptor. In this step, oxygen is compressed and dissolved into the liquid, which consumes a lot of energy and cost. The second step is anaerobic denitrification, in which the nitrate is converted into gaseous nitrogen by heterotrophic microorganism under the metabolism utilizing an organic substance as carbon source. However, wastewater usually lacks sufficient carbon source for the denitrification reaction, resulting in that operators must add extra carbon source to the reactor, thereby increasing the operation cost. Additionally, the enormous sludge yielded by the heterotrophic bacteria takes much effort to dispose of Aerobic nitrification and anaerobic denitrification processes involving two different reaction conditions need respective reactors, which increase the fabrication cost.
An improved method for treating nitrogenous wastewater, anaerobic ammonium oxidation (ANAMMOX), is also developed. As disclosed in U.S. Pat. No. 5,078,884, the ANAMMOX process under anaerobic condition is caused by autotrophic denitrifying bacteria capable of using ammonium as electron donor, using nitrite as electron acceptor, and directly producing nitrogen gas. Additionally, if nitrite is not sufficient to supply for the autotrophic denitrification, ammonium is designed to be partially nitrified into nitrite first, and then proceeds the ANAMMOX process with the remaining ammonium. Since nitrification is an aerobic reaction and ANAMMOX process is an anaerobic process, two respective reactors are also necessary to implement nitrification and ANAMMOX processes, which also increase the costs of maintenance and fabrication. Besides, the ANAMMOX process is not able to remove COD from the wastewater.